The present invention generally relates to wireless communication networks and, more particularly, to a system and method to reduce access delay associated with multiple access attempt exchanges between devices associated with such a network. Generally, wireless communications supply users with numerous services via a variety of devices, including mobile telephones, pagers, and handheld devices, gaming devices and servers. Such services may include voice, paging, video, and messaging; i.e., sending and receiving text messages via mobile devices such as mobile telephones. Advanced messaging features may include group paging, pre-programmed messages, on-call groups, scheduled messages, and repeating messages. Integrated services provide two or more services via the same device. For example, one such integrated service, push-to-talk (PTT), delivers audio messaging services in real-time, combining the flexibility of messaging with the ease-of-use of voice. PTT permits users to connect directly, in seconds, to another PTT user by pushing a button on a mobile telephone (not unlike the communication exchange between “walkie-talkies”). PTT services often include group-calling, in which icons appear directly on the user's handset, indicating which users are available to participate. This eliminates the need for the user to waste valuable time attempting to identify which of the contacts are available. Group calling further provides the advantage of providing voice services between many users at the same time, as opposed to one-on-one conversations.
The Code Division Multiple Access (CDMA) system is widely employed in many of the aforementioned services. It allows numerous signals to occupy a common frequency band, optimizing the use of available bandwidth. CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. ADC facilitates the conversion of audio input from the sending device to binary elements. The binary elements are then transmitted to the receiving device. The stream of binary elements from one mobile sending device is distinguished from the binary elements of another mobile sending device by means of Pseudonoise (PN) sequences. There are trillions of possible codes generated from the Pseudonoise sequence, thus minimizing interference.
The varied code pattern employed in CDMA applications is calculated according to a specific, complex algorithm that is a function of time. To receive signals transmitted according to this scheme, the sending device and receiving device must “know” both the code generating function and its current position in the sequence. Therefore the sending device and the receiving device must initially synchronize respective time clocks to ensure simultaneous starting time points. Synchronization of the respective time clocks typically includes turning on the transmit clock at the correct frequency and loading the correct state into the long code generator and I-channel and Q-channel PN sequence generators at the correct time to ensure that codes output by the long code generator and the I-channel and Q-channel PN sequence generators have the proper sequence at the proper time; i.e., synchronized to CDMA system time. References to “PN generators,” as used herein, refer to both the long code generator and the I-channel and Q-channel PN sequence generators, which are clocked by the transmit clock. According to the CDMA IS2000 standard, the CDMA long code generator is the CDMA system time PN sequence generator. This long code is used to channelize the mobile to decrease interference and eliminate cross-talk. If the sending device and receiving device have their long codes aligned differently in time, then communication will fail. The I-channel and Q-channel PN sequence is used for the transmit time reference.
As a precursor to many CDMA applications, an initial access process consisting of several asynchronous access attempt messages, or multiple access attempt exchange (MAAE), may be executed between the sending device and an initial receiving device, such as a base station system, or equipment designed to initially receive signals transmitted from the sending device. For example, in PTT applications, once the user invokes the PTT service by pushing a button on the mobile telephone, the MAAE commences. An MAAE of the prior art typically entails drawing current from a battery of the sending device to provide power to a transmit clock associated with the sending device; synchronizing the transmit clock, with its associated long code and I-channel and Q-channel PN codes, with a CDMA time source associated with the communication network; attempting to access the receiving device via the communication network to establish a link; i.e., sending a series of coded signals or probes; then removing current flow from the battery to the transmit clock (which results in stoppage of the transmit clock) to conserve energy and prolong the life of the battery. The foregoing steps are iteratively repeated in rapid succession, as more than one access attempt is generally required during a single MAAE before successful establishment of a link occurs. A classic example of an MAAE occurs each time a user presses the button of a mobile telephone to initiate a push-to-talk session.
Users of mobile communication services, such as PTT, expect a seamless, near-instantaneous communication connection between their device, such as a mobile telephone, and another device, such as the mobile telephone of another user. Despite such expectations, mobile communication services of the prior art typically incur significant delays, particularly during the initial connect phase, or the MAAE. Such delays are compounded by the time lost each time the battery current is reapplied to the transmit clock and the sending device must synchronize its transmit clock and associated PN generators with the CDMA time source.
One alternative to reduce transitional delay during MAAE is to permit the battery current flow to the transmit clock to remain activated for the entire time the sending device is powered on whether or not an MAAE is in progress, thus eliminating the need for subsequent synchronization processes after the initial time synchronization. This alternative quickly expends power resources and adversely impacts battery life.
It is apparent from the foregoing that a need exists for an improved system and method for minimizing delay during the MAAE phase in wireless communication applications. There is a further need to provide such a system and method that conserves energy resources and prolongs battery life.